add function single generic type support

[uraneko]

PR Type

Other(improvement)

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

Closes #2866

[JohnTitor]

Thanks for the PR! I've tweaked to support more generics and have better test suite.
But the current implementation has technically breaking change around Default impl although I think the current is one of good form to land.
If you have a good idea here, I'm happy to hear and see it :)